Fix storyboard segues on iOS < 6.0

According to the stack trace below, UICollectionView is responsible for starting segues in iOS 6.

![bt](https://f.cloud.github.com/assets/853858/299463/cbece5bc-957b-11e2-824c-a0968b27226a.png)

And the corresponding UICollectionViewCell is an argument to 
[-[UIStoryboardSegueTemplate perform:]](https://github.com/nst/iOS-Runtime-Headers/blob/master/Frameworks/UIKit.framework/UIStoryboardSegueTemplate.h)

![args](https://f.cloud.github.com/assets/853858/299464/d22d9534-957b-11e2-9fc3-e011bbd77ba9.png)

Unfortunately it's not the case for PSTCollectionView, so I've tweaked it slightly ;)

PS:  And yeah, I also hate these performSelector things. But this one looks unavoidable.